The present invention relates to a two-stroke cycle gasoline engine, and, more particularly, to a two-stroke cycle gasoline engine adapted for use with automobiles.
A two-stroke cycle engine has theoretically the advantage that an engine of a certain size can generate a greater power than a four-stroke cycle engine of a bigger size because the two-stroke cycle engine has twice as many work cycles per revolution as the four-stroke cycle engine. In fact, however, a conventional two-stroke cycle gasoline engine employing a carburetor has drawbacks, such as: that it has high fuel consumption as compared with a four stroke-cycle engine due to the loss of fuel-air mixture caused by the direct escape, i.e. blow-out, of scavenging mixture to the exhaust manifold during scavenging; that it cannot generate such a high power as expected from the fact that it has twice as many work strokes as the corresponding four-stroke cycle engine, due to the fact that the scavenging is still insufficient; and that it is subject to unbalanced combustion of fuel in the cylinders, due to the large amount of exhaust gases remaining in the power cylinder because of incomplete scavenging. This unbalanced combustion of fuel due to incomplete scavenging is particularly liable to occur when an engine which is designed to provide a high power output is operated in the idling condition or at a relatively low partial load, as is often the case with automobile engines. If such unbalanced combustion occurs, the engine generates a high level of vibration and noise, and its fuel consumption further deteriorates, together with increase of HC in the exhaust gases.
Methods of scavenging in two-stroke cycle engines are conventionally known as cross scavenging, loop scavenging, and uniflow scavenging. In this connection, if the amount of scavenging mixture is increased so as to improve scavenging efficiency, uniflow scavenging is considered to be most desirable, in order to obtain the highest scavenging efficiency without causing direct escape of the scavenging mixture to the exhaust manifold. In view of this, and in view of the aforementioned drawbacks, the actual application of two-stroke cycle gasoline engines has been conventionally limited to the field of small-size engines in which simplicity of structure and low manufacturing cost are essential conditions. Therefore, conventional two-stroke cycle gasoline engines presently used generally employ crankcase compression for scavenging. However, scavenging by crankcase compression cannot deliver a sufficient amount of scavenging mixture, thereby causing incomplete scavenging which leads to a relatively low volumetric efficiency.
In view of the fact that such a low volumetric efficiency is the principal cause of the poor output power of conventional two-stroke cycle gasoline engines, in a co-pending U.S. patent application Ser. No. 917,244 we have proposed a two-stroke cycle gasoline engine particularly suitable for use as an automobile engine, which comprises at least one two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and two horizontally opposed pistons, and at least one scavenging pump cylinder-piston assembly of the reciprocating type, with or without incorporating crankcase compression, wherein the total stroke volume of the scavenging pump means is 1.35 to 1.85 times as large as that of the power cylinder-piston assembly, so that the volumetric efficiency is substantially increased so as to generate high power output when compared with conventional two-stroke cycle gasoline engines.
Furthermore, in view of the fact that, even when a separate pump cylinder-piston assembly is employed as proposed in the abovementioned former application, if the conventional crankcase compression is also incorporated, the operational phase relation between the power cylinder-piston assembly and the scavenging pump means is substantially restricted, we have proposed in our U.S. Pat. No. 4,185,596 not to utilize at all crankcase compression, and to provide a two-stroke cycle gasoline engine piston assembly incorporating uniflow scavenging and two horizontally opposed pistons, and at least one scavenging pump cylinder-piston assembly of the reciprocating type and driven by said power cylinder-piston assembly in synchronization therewith with a phase difference therebetween. The total stroke volume of the pump cylinder-piston assembly is between 1.15 and 1.65 times as large as that of the power cylinder-piston assembly, and the phase difference between the power and the pump cylinder-piston assemblies is so determined that the top dead center of a pump cylinder-piston assembly is, as viewed in the crank angle diagram, in a range between 15.degree. in advance of and 15.degree. behind the midpoint between the bottom dead center and the scavenging port closing phase point of the power cylinder-piston assembly to which it supplies scavenging mixture. This structure substantially improves scavenging efficiency when compared with conventional two-stroke cycle gasoline engines, so that the engine can generate high output power and is suitable for use as an automobile engine.
In either of the abovementioned formerly proposed two-stroke cycle gasoline engines, in order to improve scavenging efficiency, the amount of scavenging mixture is increased by employing an additional or separate pump when compared with the conventional scavenging dependent only upon crankcase compression. In this connection, it is noted that a two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and two horizontally opposed pistons generally provides a relatively long distance between its scavenging ports and its exhaust ports when compared with two-stroke cycle engines incorporating other types of scavenging, and that, particularly when such an engine further incorporates an additional or separate scavenging pump means so as to increase the amount of scavenging mixture when compared with the conventional two-stroke cycle engines dependent upon only crankcase compression, as in the aforementioned formerly proposed two-stroke cycle engines, the engine can be constructed as a long-stroke engine having a further increased distance between its scavenging ports and its exhaust ports.
A feature in which uniflow scavenging is different from other types of scavenging is that, if necessary, it is able to perform scavenging without causing any substantial turbulence in the power cylinder. In other words, if it is so designed that scavenging mixture is introduced into a power cylinder through its scavenging ports with as small turbulence or swirl as possible being given to the scavenging mixture, the scavenging mixture then travels in the power cylinder in a stratified manner so that the scavenging mixture and the exhaust gases are transferred from the scavenging ports to the exhaust ports while maintaining stratification between themselves. If such stratified scavenging is performed, and if in this case the scavenging mixture is a mixture of gasoline and air, this mixture is decomposed by heating applied by the exhaust gases in the bordering area between the mixture and the exhaust gases so as to generate chemically activated radicals such as C2, CH, CHO, OOH, H, etc..
In order that such a decomposition of fuel-air mixture is effectively performed, not only a mass of exhaust gases having a high temperature and a high heat capacity is required, but also it is necessary that fuel-air mixture contacts such a mass of exhaust gases at their bordering area without substantially disturbing the mass of exhaust gases, because, if the fuel-air mixture and the exhaust gases substantially mix with each other so that the mass of exhaust gases is disturbed, the exhaust gases are cooled somewhat and fall below the high temperature which is required for effecting the said decomposition of fuel-air mixture. In this connection, in uniflow scavenging it is possible to perform scavenging without causing great mixing between scavenging mixture and exhaust gases, so that they are maintained in a stratified condition, and particularly in an engine which incorporates uniflow scavenging and two horizontally opposed pistons the distance between the scavenging ports and the exhaust ports is made particularly long, whereby the time during which scavenging mixture and exhaust gases contact each other in a stratified condition so that the mixture is decomposed so as to generate activated radicals is longer, thereby increasing the amount of radicals generated.
Therefore, in connection with the concept of employing a two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and two horizontally opposed pistons as a power cylinder of a two-stroke cycle gasoline engine, it is contemplated to give as little turbulence as possible to the scavenging mixture introduced into the power cylinder so as to perform scavenging of the power cylinder while maintaining as good a stratified condition as possible between the scavenging mixture and exhaust gases, so that the mixture is decomposed by the high temperature and heat of the exhaust gases so as to generate a large amount of activated radicals and so as to improve combustion of fuel in the power cylinder.
However, in connection with the fact that the present invention intends to provide a two-stroke cycle gasoline engine particularly suitable for use as an automobile engine, it is noted that the abovementioned generation of radicals due to stratified scavenging is effective for improving combustion of fuel when the engine is idling or operating at partial load, thereby effectively solving problems such as high vibration and noise, poor fuel consumption, increase of HC in exhaust gases, etc., due to unbalanced combustion caused by misfiring in idling or low load operation, while on the other hand when the engine is operating at high load the amount of scavenging mixture as well as the rotational speed of the engine increases so that the scavenging speed must be correspondingly increased to such an extent that the decomposition of mixture by exhaust gases is no longer effectively performed. In this operating condition, in order to increase the combustion speed of mixture, it is more effective to give turbulence positively to the mixture so that fuel is well mixed with combustion air. As a result of experiments, we have found that in low load operation at delivery ratio under about 0.4 decomposition of mixture by stratified scavenging is effective for improving combustion of fuel so as to improve fuel consumption by 10%-20% and to reduce concentration of HC in exhaust gases to about 1/3 when compared with conventional two-stroke cycle gasoline engines, while, on the other hand, in medium to high load operation at delivery ratio at or above approximately 0.4, it is more effective for improving combustion of mixture to give a proper swirl to the flow of mixture introduced into the power cylinder through the scavenging ports so as to generate turbulence in the power cylinder so that fuel-air mixture is strongly agitated. The term "delivery ratio" refers to the ratio of the weight of the total amount of scavenging mixture used in one scavenging of the power cylinder to the weight of scavenging mixture at standard atmospheric conditions which fills the stroke volume space of the power cylinder.
In this connection, in a two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and two horizontally opposed pistons wherein a long distance is available between the scavenging and the exhaust ports, if a good stratified condition is maintained during scavenging in low load operation at delivery ratio below approximately 0.4, decomposition of fuel-air mixture can proceed so far as to cause automatic ignition by compression. On the other hand, when a proper swirl is given to the flow of scavenging mixture, particularly in a two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and two horizontally opposed pistons, in medium to high load operation at delivery ratio at or above approximately 0.4, the swirl of scavenging mixture is well maintained during the compression stroke, and contributes to accomplishing quick and desirable combustion of fuel after ignition.
Ignition rate of fuel-air mixture in idling operation of two-stroke cycle gasoline engines is substantially lower than that of four-stroke cycle gasoline engines, and, because of this, two-stroke cycle gasoline engines have the drawbacks that they generate high noise and vibration and discharge exhaust gases which have high HC content and an offensive odor. When an engine operates in an irregular combustion mode with occasional misfiring and irregular combustion of fuel-air mixture in a cylinder, as a matter of course the fuel consumption deteriorates. The irregular combustion which occurs in two-stroke cycle gasoline engines is due to insufficient scavenging of the power cylinder, and this is more apt to occur in idling operation in which only a very small amount of scavenging mixture is available. In idling operation, the aforementioned decomposition of fuel-air mixture hardly occurs because the heat capacity of the remaining exhaust gases is very small. Therefore, in order to avoid occurrence of irregular combustion of the engine, it is desirable that the scavenging mixture should be blown into the power cylinder at high speed so that turbulence of the mixture is generated in the power cylinder. This turbulence will blow off the combustion gases remaining around the ignition plug, thereby improving igniting performance of the ignition plug, and will prevent irregular combustion due to increase of the combustion speed of fuel-air mixture.